parse-flags-from-cli-arguments.md

Parse Flags From CLI Arguments

Though we can grab the arguments to a Go program from os.Args, it requires some manual parsing. With the built-in flag package, we can declare specific flags our program accepts, by type. When we parse them, they will be separated out from the rest of the positional arguments.

Here is an example of the program that accepts a boolean debug flag. This will work with either -debug or --debug.

package main

import (
	"flag"
	"fmt"
	"os"
)

func main() {
	var debug bool
	flag.BoolVar(&debug, "debug", false, "turns on debug mode, extra logging")
	flag.Parse()

	positionalArgs := flag.Args()

	if len(positionalArgs) < 1 {
		fmt.Println("Please specify which part to run: 1 or 2")
		os.Exit(1)
	}

	if debug {
		fmt.Println("We are in debug mode...")
		fmt.Println("Received the following argument:", positionalArgs[0])
	}

	// ...
}

We can run the program in debug mode like so:

$ go run . --debug 123
We are in debug mode...
Received the following argument: 123

We can also take advantage of the help flag that we get for free:

$ go run . --help
Usage of /var/folders/62/lx9pcjbs1zbd83zg6twwym2r0000gn/T/go-build3212087168/b001/exe/test:
  -debug
        turns on debug mode, extra logging

Note: any recognized flags need to come before any of the position arguments. The debug flag won't be picked up if we run the program like this:

$ go run . 123 --debug

source